In recent years, as countermeasures against global warming, improvement of fuel economy of an automobile has been demanded from a viewpoint of regulating a greenhouse effect gas emission amount. Accordingly, in order to realize reduction in weight of a vehicle body and to secure collision safety, a high-strength steel sheet is increasingly applied. Further, to a portion required to have rust prevention, an ultrahigh-strength steel sheet which is subjected to hot-dip galvanizing is required to be applied.
Particularly, in recent years, needs for an ultrahigh-strength steel sheet and an ultrahigh-strength hot-dip galvanized steel sheet having a tensile strength of 1300 MPa or more is increasing. Besides, to a member required to suppress deformation at a time of collision, an ultrahigh-strength steel sheet having a high yield ratio is required to be applied.
However, when applying an ultrahigh-strength steel sheet whose tensile strength exceeds 1300 MPa, there is a need to solve hydrogen embrittlement of the steel sheet. The hydrogen embrittlement is a phenomenon in which a steel member to which a high stress is applied in a situation of being used is fractured by an additional stress being equal to or less than a maximum tensile stress due to hydrogen which enters from an environment.
Generally, as a tensile strength of a steel sheet increases, the hydrogen embrittlement resistance of the steel sheet deteriorates, and its mechanism itself has not been clarified yet.
Various attempts to improve the hydrogen embrittlement of steel sheets have been made so far. Examination cases thereof will be described below.
Patent Literature 1 discloses a technique regarding a high-strength steel sheet which realizes both of high strengthening and hydrogen embrittlement resistance in a manner that a decarburization treatment is performed on a surface layer of a steel sheet to increase a ferrite volume fraction of the surface layer of the steel sheet, which causes softening, a structure inside the steel sheet is mainly constituted of ferrite, and besides, a small amount of martensite having fine blocks is dispersed. However, the steel sheet described in Patent Literature 1 contains a considerable amount of ferrite being a soft structure, so that it is not preferable for obtaining a high yield ratio.
Patent Literature 2 discloses a technique regarding a high-strength hot-dip galvanized steel sheet which realizes both of workability and hydrogen embrittlement resistance by properly controlling an average grain diameter and an aspect ratio as a form of ferrite. However, the steel sheet described in Patent Literature 2 also contains a certain amount of ferrite being a soft structure, so that it is predicted that the steel sheet is not preferable for obtaining a high yield ratio.
Patent Literature 3 discloses a technique regarding a high-strength hot-dip galvanized steel sheet which improves the hydrogen embrittlement resistance by setting a steel structure to a structure mainly constituted of martensite, and by making carbides of Nb, V, Cr, Ti, and Mo and the like to be precipitated and making the carbides function as hydrogen trap sites. However, also in the steel sheet described in Patent Literature 3, a high yield ratio is not taken into consideration.
Patent Literature 4 discloses a technique regarding a high-strength hot-dip galvanized steel sheet which improves the hydrogen embrittlement resistance by setting a steel structure to a structure mainly constituted of bainite, and by regulating retained austenite to less than 4%.
However, bainite generated in a hot-dip galvanizing process is often upper bainite due to its retention temperature region. The upper bainite is a structure with inferior toughness when compared to tempered martensite and lower bainite, so that reduction in toughness is concerned in a steel sheet having upper bainite as a main structure.